Solve the system of equations using the row echelon method. Select the correct choice below and, if necessary, fill in the answer box to complete your choice. 9x - 3y = 54 2x + y=7 O A. There is one solution. The solution set is { (Type an ordered pair. Simplify your answer.) O B. There are infinitely many solutions. The solution set is the set of all ordered pairs { (,y) }, where y is any real number. (Type an expression using y as the variable. Simplify your answer.) O C. The solution set is the empty set